Clarity on Antarctic sea ice.

I’ve always been a skeptic when it comes to Antarctic sea ice. I’m not referring here to the tiresome (and incorrect) claim that the expansion of sea ice around Antarctica somehow cancels out the dramatic losses of sea ice in the Arctic (NB: polar bears don’t really care if there is sea ice in Antarctica or not). Rather, I’m referring to the idea that the observation of Antarctic sea ice expansion represents a major conundrum in our understanding of the climate system, something one hears even from knowledgeable commentators. In this post, I’ll try to provide some clarity on this subject, with some basic background and discussion of a couple of important recent papers.

In general, Antarctic sea ice forms near the coastline, where upwelling waters cool to the atmosphere. It melts when the winds and currents push it into areas of warmer water to the north. In the summer, it melts pretty much all the way back to the coast. An efficient way to form lots of Antarctic sea ice during the autumn growth season is to have strong winds that push the ice away from the coastline. Pushing sea ice away leaves open water that can lose heat to the atmosphere, creating more sea ice. The persistent circumpolar westerlies are critical in pushing ice toward the north, into warmer waters. (Owing to the Coriolis effect, westerly winds cause northward-flowing surface ocean currents in the Southern Hemisphere).

The importance of the winds in controlling Antarctic sea ice leads to the obvious idea that changing winds can explain the increase that has been observed over the last several decades. There has indeed been a substantial increase in the circumpolar westerlies; this is very well established from observations and is associated with the oft-discussed increase in the “Southern Annular Mode” (SAM) index2. Averaged over the year, the SAM index has increased nearly monotonically since the 1970s (e.g., Marshall et al., 2003). This has led to a fairly simple logic in explaining the recent sea ice increase: the westerly winds have increased, so sea ice has increased too. Furthermore, there is good evidence that the increasing westerlies are a response to anthropogenic climate forcing from CO2 and other greenhouse gas increases in the troposphere, along with ozone declines in the stratosphere (Thompson and Solomon, 2002; Thompson et al., 2011). This would suggest that the observed increase in Antarctic sea ice extent is anthropogenic in origin, just like the Arctic sea ice decline, but for very different reasons. In short, reduced ozone in the stratosphere, and increased CO2 in the troposphere — both climate forcings that are unequivocally anthropogenic — cause increased westerly winds, which cause Antarctic sea ice to expand.

Of course, it’s not that simple. For one thing, the average increase of Antarctic sea ice is actually a small number that is the difference of two big numbers — modest increases over a large area, mostly in the Eastern Hemisphere, and very large decreases over a smaller area in the Western Hemisphere. The map below, showing change in the length of the sea ice season over the last 30 years, illustrates this point well. In spite of the average increase, there are very rapid declines in the Bellingshausen and Amundsen Seas, comparable to sea ice declines in the Arctic. Furthermore, the only season is which there is a significant trend in the westerlies is austral summer. There is a weak positive trend in fall, but both spring and winter show no trend; the SAM trends in these seasons may even be slightly negative, depending on which data are used (Ding et al., 2012). Yet the pattern of sea ice change is quite similar in all seasons: decreasing along the Pacific coast of West Antarctica, and increasing around most of East Antarctica, and in the Ross and Weddell Seas.

Trend in the length of the sea ice season, 1979-2010. Blue and purple areas show areas where sea ice is declining, orange and red where it is increasing. Source: Maksym et al., 2012

On top of these subtleties, confusion about the role of the winds has arisen because some of the prominent modeling studies that have examined the relationship between the westerly winds and Antarctic sea ice have come up with results that appear to be in direct opposition to the observations. When fully coupled climate models are run with increased CO2 and decreased stratospheric ozone, the westerly winds increase as has been observed, but sea ice decreases around most of Antarctica. For example, Bitz and Polvani, 2012 found that the pattern of trends is the mirror image of the observations, with increases, rather than decreases in the Amundsen and Bellingshausen Seas.

So what’s really going on? One idea is that changes in ocean stratification might be important. There has been a huge increase in the amount of fresh water getting into the Southern Ocean from melting glaciers, especially in the Amundsen Sea (see, e.g., the latest data from Sutterly et al., 2014). Fresh water forms a sort of buoyant lid on the ocean, limiting the ability of heat from the warmer water below to get to the sea ice and melt it. A study by Bintanja et al. (2013) showed that it was a least plausible that this explains the Antarctic sea ice change. A basic problem, though, is that the greatest discharge of meltwater is occurring in the Amundsen Sea, exactly where sea ice is declining, so while this probably is part of the story, I doubt it’s very dominant.

As it turns out, comparing observations with the results of model experiments like those of Bitz and Polvani (2012) is misleading. Most such experiments are equilibrium experiments: What’s done is to run a model under “preindustrial” conditions, and then to run it again with reduced ozone and increased CO2, and to look at the difference. This provide a measure of what will eventually happen (at least in the model) after many decades or centuries. But when you look at the transient response to changes in the circumpolar winds, as Marshall et al (2014) have done, it turns out that two important things happen. The winds tend to push the sea ice boundary northward, as we would have expected. But the winds push the surface ocean northward too, and cause a slow rise in the isopycnal surfaces (surfaces of constant density). This brings relatively warm deep water closer to the surface, eventually melting sea ice after a period of a few decades, countering the initial increase in sea ice. These results explain why equilibrium model calculations find sea ice decreasing in response to ozone forced changes in the circumpolar winds, and also why observations show the opposite. Not enough time has passed for the equilibrium response to be manifested. These results suggest that some time in the next few decades, there will reverse, and average sea ice will begin to decline.

Furthermore, there’s a whole lot more going on with the winds than just “increased westerlies”. In the areas where the big sea ice losses have occurred, the concept of “circumpolar westerlies” isn’t very relevant. A far more important measure of wind variability in the Amundsen and Bellingshausen Seas is the Amundsen Sea Low (ASL).5 The ASL describes the average location of storms systems the bring heat and moisture into West Antarctica. Changes in the ASL may occur for myriad reasons, but one big hammer that can make it ring is the propagation of atmospheric planetary wave arising out of the tropics, more-or-or less associated with ENSO (El Niño-Southern Oscillation) variability. It’s been clear for many years that ENSO variability play a significant role in sea ice variability in those regions, and recent work shows that this can explain the trends pretty well too (e.g. Yuan and Li, 2008; Stammerjohn et al., 2008). Not incidentally, the adjacent land areas of the Antarctic Peninsula and the West Antarctic Ice Sheet have warmed significantly over the last few decades (Steig et al, 2009;Orsi et al., 2013; Bromwich et al, 2013), and those changes can also be attributed largely to tropical climate variability (Schneider and Steig, 2008; Ding et al., 2011; Schneider et al., 2012; Steig et al., 2013). The cause of temperature and sea ice change is the same: more warm air is being steered into West Antarctica, and the atmospheric flow tends to push sea ice against the continent, keeping it from expanding.

So, do we get the right answer if we take into account all of the wind changes that have occurred over the last few decades? The answer is yes. This is nicely illustrated in a study by Holland and Kwok (2012), who showed that wind, ice motion, and ice concentration changes match each other remarkably well. Where the wind has been increasingly northward, concentrations are increasing; where wind and ice motion changes are toward the continent, ice concentrations are decreasing. And this year, Holland et al. (2014), showed that when they drive an ocean and sea ice model with observed winds — not just increased westerlies, but the full range of wind changes, as calculated by the ECMWF (European Center for Medium Range Weather Forecasting) –- they correctly simulate the overall expansion of sea ice, and they also get the pattern of changes pretty much spot-on. To be sure, the authors note that not all the details are explained, and they highlight the possibly greater importance of thermodynamic consideration (i.e. ocean temperature/stratification) in some areas than in others. Also, the period they study (1992-2010 only) is pretty short. The results are nevertheless pretty compelling. Just like the observations, the calculations show large decreases in the Amundsen and Bellinghausen seas, but increases nearly everywhere else.7

Taken as a whole, these results show that there is no significant contradiction between our understanding of Antarctic sea ice and the observation that it is, in average, expanding. We can explain sea ice trends in the Antarctic rather well if we take into account the full range of changes in winds that have occurred. The average expansion of Antarctic sea ice was not anticipated, but it hardly represents any sort of existential threat to our fundamental understanding of the climate system as a whole. It’s merely an interesting scientific challenge.

Not incidentally, changing winds also have a lot to do with what’s been happening to the Antarctic ice sheet (meaning the land-based glaciers, distinct from the sea ice). I’ll have another post on that later this month, or in the New Year.

108 Responses to “Clarity on Antarctic sea ice.”

You left out the pioneering work of Manabe et a from 1991 and 1992. From their first paper which suggested that with increasing CO2 concentrations, increased precipitation at higher latitudes which lowers the salinity on the ocean surface leading to greater sea ice. : http://journals.ametsoc.org/doi/pdf/10.1175/1520-0442(1991)004%3C0785%3ATROACO%3E2.0.CO%3B2
Page 813 : “In the Circumpolar Ocean of the Southern Hemisphere, surface salinity is also reduced due to the increase of freshwater flux from the atmosphere. The reduction of salinity and density in the near-surface layer, in turn, weakens the convective mixing of cold surface water with the underlying warmer water, thereby lowering sea surface temperature. This near-surface process together with the deep vertical mixing of heat trapped by increasing greenhouse gas make the warming of the sea surface very small, sustaining the regions of no warming in the immediate vicinity of the Antarctic Continent.”

[Response: Indeed, thanks for pointing this out. Much as I like the new paper by Marshall et al., it is true that most of this was understood by Manabe — like many thing in climate science, he got there first. Note, however, that I remain unconvinced this process is dominant in explaining recent trends. I could be wrong of course — going against Manabe’s insights is very often a bad bet! –eric]

[Response: Re geography, yes, that is certainly part of the reason for the spatial pattern. Lots of previous work on this, Yuan and Martinson come to mind for example (look up “Antarctic dipole” and “sea ice” and you’ll find relevant links). Re citations, I don’t think Manabe thought of everything — specifically, I don’t recall the Manabe’s papers actually talk about the transient response (increased sea ice) that Marshall et al. are talking about. –eric]

“Thwaites Glacier, the large, rapidly changing outlet of the West Antarctic Ice Sheet, is not only being eroded by the ocean, it’s being melted from below by geothermal heat, researchers at the Institute for Geophysics at The University of Texas at Austin (UTIG) report in the current edition of the Proceedings of the National Academy of Sciences. . .
The cause of the variable distribution of heat beneath the glacier is thought to be the movement of magma and associated volcanic activity arising from the rifting of the Earth’s crust beneath the West Antarctic Ice Sheet.”

It’s occurred to me that venting from undersea rifts of this kind could be contributing to oceanic heating in many regions of the globe. Do you know if this possibility has been studied?

[Response: I am 99.995% confident this isn’t important. That UTIG study is greatly misinterpreted — there is not evidence of an important *change* in the amount of volcanic activity; that paper only shows that the sub-ice heating is large. The bottom of the ice sheet cares about this because it is an important local source of heat. The ocean does not care about this — we’re talking the difference between W/m^2 from the sun, vs. milliwatts from heat from the ocean bedrock.—eric]

Thank you for this review. I’ve been reading the monthly overviews from the National Snow and Ice Data Center, but they haven’t addressed the research on the Antarctic as much as I’d like. (There was a reference several months back to the possibility of increased freshwater from melting land ice playing a role.)

[Response: You cannot do things this way. You cannot average Denmark and Iraq, and then conclude the world is therefore at peace. The minimum in Antarctica sea ice occurs in the Northern Hemisphere winter, when Arctic sea ice it at it’s maximum! Adding these things together makes no sense. I didn’t say anything about polar amplification but there is significant polar amplification in the Northern Hemisphere. The Arctic is warming about twice as fast as the global average. That Antarctica is, on average, not doing the same (though it is warming) has no bearing on that statement being true.–eric]

Thank you for this post Dr. Steig, I didn’t see where you addressed the idea that Antarctica is not in a state of irreversible decline, only that it’s adding ice in some places and losing it in others. I understand that this melting process will play out over hundreds to thousands of years but given the amount of CO2 that’s already in the atmosphere and the fact that we’re still adding CO2, do you see any way to stop or reverse the melting process? Maybe you can point to something I missed.

Thanks

[Response: Chuck: I didn’t address that idea because this was about Antarctic sea ice, which is NOT declining. The Antarctic ice sheet — a totally different thing — is losing mass, but only in some areas. Whether this is an irreversible decline is debatable; for my part, I don’t think the evidence is there, at least not yet. I’ll write more about that later. –eric]

Thanx for the article, Prof. Steig. Holland(2014) is quite clean. I note the increased thermo effect off Amery, not something that was otherwise obvious to me. I look forward to your thoughts on Antarctic land ice.

OK, I get it. Where ice is reducing, such as in the Arctic, it’s due to manmade global warming – not natural. But where the ice is expanding, such as the Antarctic, there’s another explanation, based on natural influences. I like the way Climate Science is transparent and simple.

oakwood, did you actually read the article? Where Eric describes how anthropogenic ozone and CO2 are causing (transient, perhaps a few decades) wind changes leading to the current Antarctic sea ice expansion?

Because your comment shows no sign of actually following the discussion.

Tegeri – you lost me. The increase in Antarctic sea ice happens when there isnt much sun to affect the albedo. In the Arctic, the decrease happens when summer which the change maximizes the effect on albedo. I am lost as how you think Tamino’s insolation calculation supports your argument rather than the exact opposite.

There are a lot of natural factors contributing to the long-term decline in Arctic sea ice, oakwood, but rising CO2 and black carbon are not two of them. Your snark does not make up for for your lack of understanding of the topic.

As a novice here (great website BTW) I’m wondering if the changes in wind patterns we’re seeing in the Arctic winter, where the circumpolar winds seem to be destabilizing due to GW, have the same basis as the stronger Westerlies around the southern pole.

I ask mainly because my denier brother-in-law is coming for Christmas and I want to be able to answer intelligently.

One suspects oakwood is a drive-by, and will ignore our indignant reactions. It may, however, offer insight into a segment of the “skeptical” public that believes science has to be “simple”, whatever that means, or it can’t be true. I wonder if that’s a uniquely American (or possibly Anglophone) supposition?

“a segment of the “skeptical” public that believes science has to be “simple”, whatever that means, or it can’t be true.”

All too true. In a discussion I once had with a “skeptic” they used the common assertion that climate science is not a real science, like physics and chemistry are. When I pointed out that climate science basically is physics and chemistry, plus biology, astronomy, geology, oceanography, etc.. etc. they just sneered. At another point they asserted that science is all about proven facts, theorems and laws. Sounded like they had never taken any science beyond high school, or even grade school, where the focus is on proven facts, theorems and laws so that one has the foundation for going further in the study of science. It never occurred to them that the very purpose of science is to push our knowledge beyond what is already known and established.

The changing albedo influence should be aggregated for the whole year. The crux of Tamino’s argument is that the poles get more solar energy during summer peak than any other place on Earth. However, consider a typical solar radiation distribution:

At fig 8 witness that 80 degree latitude daily solar radiation maximum in June is perhaps 10% higher that the one at 60 degree, but notice how wider the 60 degree latitude graph is. Therefore, I stand by the assertion that increased sea ice in the lower latitudes in the Antarctic should have greater effect on albedo than decreased sea ice in the Arctic.

[Response: There is no virtually no sea ice north of about 65°S in summer, and there is very little trend, so I’m not sure what your point is? Tamino’s calculation is shown below — this shows the total solar energy incident on sea ice. The loss in the Arctic clearly exceeds the gain the Antarctica, by about a factor of six. –eric]

Furthermore, there is good evidence that the increasing westerlies are a response to anthropogenic climate forcing from CO2 and other greenhouse gas increases in the troposphere, along with ozone declines in the stratosphere

A paper was published last week in GRL as well which contains evidence of the same, It’s called Anthropogenic influence on recent circulation-driven Antarctic sea-ice changes by Haumann et al.

“But even though the model simulates a lowering of the surface pressure, it does not get the ice increase. Also this the scientists now understand: They suspect that the model does not capture the influence of the smaller scale topography around the continent and surface processes over ice and snow accurately enough. These processes influence the surface-pressure distribution and hence the direction of the wind. “In our model, the pressure changes such that the wind primarily blows stronger parallel to the coast line rather than away from it. Once this is better represented in the model, we should get better simulations of the sea-ice trend”, concludes Haumann.”

[Response: I don’t quite understand that quote. Winds parallel to the coast are what you want, if you want to push ice north. Coriolis! Anyway, here is link to the paper: Haumann et al., 2014.–eric]

Re #27 Response:
Tamino, it would be nice if you could update your plots from 2012 to the present, both for Sea Ice Insolation and for “Poles Apart” Arctic and Antarctic Sea Ice Maximum and Minimum, and do another posting pulling them together.

Sorry if I seem to be lazy, asking you to do it, but I figure if you were able to dig out the data before, you know where to find it to do the updates.

sidd (13), at this moment a satellite photo from that perspective would show pretty much the whole continent and most of the surrounding ocean, with just a slice missing off to one side. I’m rather surprised that someone at JPL wrote “While a spacecraft could find itself directly over the Earth’s pole, roughly half of the image should be in darkness!” Well, at times. Anyway, happy solstice!

Thanks for the post, Eric. As it happens I attended AGU to gather material for posts at the Arctic Sea Ice Blog, spending most of my time working through cryosphere and paleo posters. Among many other things, I saw a very interesting Antarctic sea ice poster that the authors claimed to be a solution and which looked good to my amateur eyes, but now you’ve got me wondering since it focused on meltwater effects. You’ve also saved me the trouble of getting up to speed on recent results before tackling my own post, although there remains the slight matter of reading all the papers you linked. :) After I’ve gone through everything, if I still think there’s something to it I’ll be in touch with you.

Dave (20), yes. Expansion of the tropics is compressing the entire atmospheric circulation poleward (in both directions). That’s as bad as it sounds. It’s hard to measure, but per the linked article the rate is between 1.25 – 2.5º, or 138 – 277 km, per 25 years. Also, the center of circulation (the ITCZ or Inter-Tropical Covergence Zone) is moving north. IIRC the only component that hasn’t yet shifted much is the southern polar jet.

Is ozone depletion/ the ozone hole adding to warming in the Southern Hemisphere? Or slowing down warming? Or is there a double effect? Or is it just adding to extreme weather?

Just looking for some clarity. The “it’s ozone depletion” meme comes up occasionally from deniers on line, and I have not got the time to trawl through the papers myself.

[Response: There is a huge literature on this, including the Bitz and Polvani papers and Marshall paper that I cited and linked to. Ozone depletion is certainly playing a role in SH climate change, though I think it is often overstated. It only matters demonstrably in summer and the impact (over Antarctica at least) is cooling, not warming. The Marshall et al paper does convince me it contributes at least partly to the sea ice expansion. Incidentally, the subject of ozone and Antarctic climate change was one of our earliest posts at RealClimate, almost exactly 10 years ago.–eric]

It makes for a nice, informative visualization of what matters in Antarctica. In particular, note how the low-pressure centers (red), rather than the circumpolar westerlies, dominate the wind field near Antarctica. –eric]

oakwood is one of the regular denialist trolls on the Guardian newspapers articles on global warming and other environmental issues since 2009.
He is notable for being willfully ignorant and just plain thick.
He has his misconceptions corrected ad nauseam, and still comes back for more.
I suspect that he is unemployed.

It’s perhaps worth noting that, for all its beauty, the animation at earth.nullschool is just a snapshot animation. It animates the winds or ocean currents at a particular selected instant; it does not show their progression over time (except by creating a new animation for a new time).

…”We can explain sea ice trends in the Antarctic rather well if we take into account the full range of changes in winds that have occurred”…

When then, shall this understanding reach the GCMs that are tasked with predicting Antarctic Sea Ice trends? In stocks and finance, it is often that an underperforming model be defended with a proper explanation of the recent past. But as Paul Krugman has often stated-as-much, “if your model is failing, change the model”.

When model trends are ‘mirror-opposite’ reality, when can it move beyond “interesting scientific challenge”? If it’s just that its too hard to put everything into a super-duper powerful model, mainframe time, we do as good as we can, first approximations, etc. etc., then how still can the model be relied on for an accurate representation of Antarctic Sea ice and any/all knock-on effects? If it’s that the same model is still going to be right ‘in the long run’, then that’s the same sort of thing folks have been saying about a looming monetary inflation problem, to much denigration by economic experts. (Not same field, but similar conversations when comparing/defending various models to expectations).

Thanks for the pertinent settings and what to look for. One can see how this complex slice of winds can impact the coast in spots, pull off of it in others, or drive straight out from it in a particular place, depending.

I am not a person worthy of being a regular poster here. I’m here with a question.

Could someone comment if there are effects of “freezing-point depression” are in play on the “fresher” melt waters at the leading edge ice sheets – adding to the expansion of the ice sheets by freezing the fresher melt water first at higher temps and faster? Thereby, driving the edge in a self-feeding way.

Please add a comment on: if the fresher water might be being forced to the leading edges of the ice sheets by the layered float barrier of the fresher melt waters over the normal saltwater. Is anyone measuring the mix of this melt water as it travels outward from Antarctica under the sheets (understanding there may be added melt water along the way from the ice sheet itself – caused by warmer ocean waters transferring heat and other melting causes.) Might this too be a factor in the observed expansion – when coupled with the above question?

Hope this is not a dumb question – haha

[Response: Meltwater coming off an ice sheet is often at the base of the ice. For ice shelves, the base can be hundreds of meters below the sea surface, and therefore is at relatively high pressure. Since the freezing point of sea water gets lower with pressure (about 0.075ºC less per 100 meters), that meltwater often comes out at below the surface freezing point (i.e. at around -2.1 — -2.3ºC, surface freezing point is around -1.8ºC). When it mixes with ambient sea water, it can spontaneously freeze in situ and you can get what’s called ‘marine ice’ formation (which then floats to the surface or to the underside of the ice shelf). I’m not sure this is a big enough effect though to impact the climate though – though yes, it does get measured. – gavin]

Thanks for the straw-man deflection… I’ll pass. I asked a real question. Don’t act like models (specifically models regarding Antarctic Sea Ice) have not be relied upon to characterize a future warming world, including decadal increments. My “mirror opposite” quote is right from the above post. It’s not hard. Why not concede that if the model at-present produces the opposite results from what we know (and can explain) to be true, that at a minimum the model isn’t getting something right. And, beyond the extent that ‘models are never perfect’, why not concede that predictions of Antarctic sea ice and all its knock-on effects is an area of potentially confounding uncertainty (that impacts what can be concluded), more so than it just being “interesting”?

This still may not even change much in the larger overall picture of anthroprogenic global warming, other than perhaps putting a “potential” of a little time back on the doomsday clock, though even that’s not certain. Would that be so bad..?

For those interested in that WG1AR5 Chap 9 link, some relevant summary material is located on page 744 and 790 (9.4.3). The multi-model mean does simulate that there is a seasonal cycle in sea ice, but it’s trending a decrease in extent whilst the observations are going the other way. There is no discussion in those sections on how/if this might be significant in the overall regional+ picture.